hwdec_vulkan: fix partial failure edge cases in error paths

This is another file where the slopbot identified a problem, but I was
surprised to see it fix it in a poor way and also miss an adjacement
problem.

`mapper_map` calls `mapper_unmap` with the intention of using it to clean
up partial failure after some planes are wrapped and released while others
are not. But the current code would actually segfault in that situation.

There are two fixes required:

* Ensure `p->vkf` is set by `mapper_map` early so that its available to
  `mapper_unmap` in the error path
* Don't call `vkfc->unlock_frame` in the `mapper_map` error path, as
  this will be called by `mapper_unmap`

Fixes #17977